Juan Manuel Marquez, right, said he threw “a perfect punch” that knocked out Manny Pacquiao in the sixth round Saturday night. If postfight comments from both fighters and promoter Bob Arum were any indication, Pacquiao and Marquez will more than likely fight for a fifth time.

LAS VEGAS — The idea of Manny Pacquiao being knocked out cold was shocking enough. The sight of him face down on the canvas, unresponsive even as bedlam broke out all around him, was positively frightening.

Mitt Romney saw it up close from his ringside seat just a few feet away. So did Pacquiao’s wife, who broke down in tears and tried to get in the ring to aid her downed husband.

Juan Manuel Marquez didn’t even bother to look. He was already busy celebrating the knockout of a lifetime.

This was boxing at its brutal best, a toe-to-toe slugfest Saturday night that was destined from the opening bell to be decided by fists instead of judges. Both fighters had been down, and both fighters were hurting when Marquez threw a right hand off the ropes with a second left in the sixth round that could be felt all the way in the rafters of the MGM Grand Arena.

It will go down among the great fights of their era. But it was barely over when the cry arose for the two ever-so-willing warriors to do it again.

When it comes to Pacquiao and Marquez, four fights may not be enough.

“If you give us a chance, we’ll fight again,” Pacquiao said. “I was just starting to feel confident, and then I got careless.”

Indeed, the case could be made that Pacquiao was on the verge of a big win when Marquez landed the punch that sent him falling face first on the canvas. He had come back from a third- round knockdown to drop Marquez in the fifth and was landing big left hands that broke and bloodied the Mexican’s nose.

After three fights that all went the distance both fighters had vowed to be more aggressive in their fourth meeting. Pacquiao ended up paying the price for it when he tried to close the sixth round with a flurry, a big mistake against a counterpuncher.

Pacquiao, who hadn’t been stopped in a fight since 1999 in Thailand when he was a 112-pounder, took several minutes to come around on the canvas before being led to his ring stool.

He was taken to a hospital for a precautionary brain scan, then went to his hotel suite, where he ate with his wife, Jinkee, and his entourage and watched a replay of the fight.

“Spoiler alert,” Pacquiao said as the fight played on the TV. “I don’t think you are going to like how this ends.”

Pacquiao had dropped Marquez four times in their first three fights, but Marquez had never put him down before he landed a big right hand in the third round for his first knockdown. The power was sure to raise questions about the new, bulked-up physique Marquez has at the age of 39, which he said came from hard work under a strength conditioner who once provided steroids to Marion Jones and other track stars.

Still, it was a career-defining moment for Marquez, who claims he was robbed by the judges in the first three fights. Marquez was losing by one point on all three scorecards when he landed his big punch.

KANSAS CITY, Mo. — Tyreek Hill had touchdowns receiving and on a punt return, Kansas City’s defense made life miserable for Oakland quarterback Derek Carr, and the Chiefs beat the Raiders 21-13 on a frigid Thursday night to take control of the AFC West. Charcandrick West also had a touchdown run for the Chiefs (10-3). They moved into a first-place...